Technically, it’s not every four years.
Julius Caesar introduced the idea of the first leap year around 46 B.C, but his math on how often it happens was a little off. Instead of being solely defined as any year evenly divisible by four (his words, not ours), the century years also need to be evenly divisible by 400. For example, 2000 was a leap year, but 1900 was not.
Birthdays can be existential.
Called “leaplings,” those born on February 29 usually either celebrate on February 28 or March 1 on off years. (Also, they don’t find the “you don’t have a birthday this year, so we can skip the presents” joke funny.). February 29 counts legally on all documents such as licenses and passports. The chances of having a leap birthday are one in 1,461.